Rheometer



Jan. 13,1953 w. PATTERSON, JR. 2,625,034

RHEOMETER Filed Dec. 29; 1950 INYENTOR ILLZ;M PATTERSON JR- 'T 1% ATTORNEY 0 mm (GRAMSN Patented Jan. 13, 1953 RHEOMETER William Patterson, Jr., Lancaster, Pa., assignor to Armstrong Cork Company, Lancaster, Pa., a. corporation of Pennsylvania Application December 29, 1950, Serial No. 203,285

4 Claims.

cover the surface, but the consistency should be such that one color will not bleed into another color at the zone where two colors are adjacent in the printing of fioor and wall coverings having a decorative design comprising a plurality of colors. Also, the consistency and drying properties of the paint should be such that it does not run when the coated material is hung in a vertical plane in a maturing chamber. For example, in the printing of fioor covering, the design is applied to the base by means of flat block printing 4 machines in which the individual blocks have a plurality of small pegs to which paint is applied, and the paint is then transferred from the pegs to the base material. It is essential in carrying out this operation to use a paint which has the ability to fiow so that the finished surface will be substantially level and continuous and the exact position of the indentations made by the pins will not be noticeable upon close observation of the surface.

In the past, it has been difficult to evaluate rapidly the consistency of pigmented systems (paint, printing ink, adhesives, etc., in which a solid phase is dispersed in a liquid phase). The only way that could be accomplished was to actually try the paint on the print machine or on small blocks duplicating the printing machine process. This has been an unsatisfactory system, inasmuch as it requires a great deal of time and also a considerable loss of material in making sufficient factory runs to determine whether or not a specific formulation is suitable for the purpose intended. In order to determine the suitability of the pigmented system or viscous liquid expeditiously, I have developed an instrument for making rheological measurements of the pigmented system or viscous liquid. With this instrument it is possible to study the behavior of the viscous liquid under the influence of extremely small shearing forces. Such forces are generated in a wet paint film immediately after application and cause the paint to fiow in such manner as to result in a smooth, level, and continuous film, preferably without paint runs or intermingling of different colors at their zone of 2 juncture. The latter stipulations are directed more to controlled fiow than continuous fiow up to the time the paint sets.

It has long been desired to make rheological measurements of pigmented systems at the low shear rate and shear stress levels encountered in the leveling processes, but instrumental limitations have heretofore prevented other than qualitative observations to be made.

It is the object of this invention to provide an instrument with which it is possible to make rheological measurements of pigmented systems at a very low shear rate and shear stress level, and to be able to determine accurately whetheror not the pigmented system being measured has the desirable flow characteristics.

In order that this invention may be more readily understood, it will be described in connection with the attached drawing in which Figure l is a view of the instrument developed to make the rheological measurements; and

Figure 2 shows a chart of the results obtained by using the instrument shown in Figure 1.

Referring to Figure 1, there is shown a conventional chainom-ati-c analytical chemical balance designated by the numeral 2. This balance is provided with a cross arm 3 on one end of which is suspended a weighing pan 4. On the other end of the arm 3 is suspended a cylinder or bob 5. The bob 5 is suspended in a larger hollow cylinder 6. Positioned between, but not in contact with, the arm 3 and the bob 5 is a linear differential transformer I, such as that made by Schaevitz Engineering or Sperry Gyroscope Company. The one end of the armature 8 of the transformer 1 is connected to the arm 3 by means of the wire 9. The other end of the armature 8 is connected by means of the wire [0 to the bob 5.

The bob 5 and the cylinder 6 form -a modification of the coaxial cylinder type of viscometer. The concentric cylinders are arranged so that an axial translation of the inner cylinder occurs with respect to the fixed outer cylinder, subjecting the liquid filling the annular space between them to shearing stress. The resistance offered by the liquid to the movement of the inner cylinder determines its viscosity for any constant applied force, and hence its velocity is a measure of the viscosity or resistance to flow of the liquid being sheared.

The linear differential transformer has a freely moving armature which moves in a vertical direction when the bob is moved in a vertical direction. The displacement of the armature is converted into an electric signal, the magnitude of which is in proportion to the displacement.

The positioning of the transformer is such that the armature is centered in the secondary winding at the time the analytical balance is at its rest position. At this time the electromotive force of the two halves of the secondary winding will be the same and the signal will be at a minimum. Connected to the transformer I is an amplifier and rectifier I I which amplifies the signal before transferring the same to the recorder l2. The recorder I2 is provided with suitable chart paper which travels at a constant speed, and the indicator or recording pen engages the paper, and any movement of the indicator caused by the signal from the transformer is recorded on the chart paper. The energizing current to the transformer I is supplied by the oscillator I3.

The chainomatic analytical balance and the differential transformer constitute an essentially friction-free mechanical suspension which permits a sensitivity of measurement of shear stresses which exceeds that of any known instrument of its type. It is necessary to provide such a friction-free system to carry out this invention.

In testing a specimen of paint, for example, the paint is placed in the receptacle 6 with the bob suspended therein. Weight are placed on the pan 4 until the two are in balance with the ointer of the analytical balance at rest. With the balance in this position, the signal from the transformer 1 will be at a minimum and the indicator pen of the recorder I2 will be at 0, or any allowable starting point. Weights are then added to the pan 4 in small increments, the size of the increments depending upon the consistency of the paint being measured. After each increment of weight is added, the system i left undisturbed for a period of two to six minutes. During this time interval the recorder pen will trace a curve on the chart paper, the greater the load added to the pan, the lower the slope of the curve for a given liquid. The reason for allowing the time interval of two to six minutes is to ascertain if the curve drawn is a straight line. Unless a straight line is obtained, the data will be misleading. Once it has been determined that the curve has become linear, an additional load may be added to the weight pan. If the bob has traveled sufficiently far to drive the recorder pen off the chart paper, the pen may be returned to its position of rest by adjusting the position of the transformer with respect to the armature. A screw mechanism I4 permits this adjustment to be made. This can be done as often as required without affecting the motion of the bob in the liquid until the balance beam strikes the stops I5. This permits a bob movement of approximately .2

The apparent viscosity of the sample will, of course, dictate the chart speed, the amount of signal attenuation, and the increments of load to be used. A very viscous material will permit relatively large increments of load to be applied and still obtain rather small bob velocities.

When a sufiicient number of load increments have been added and their respective curves obtained, the chart is withdrawn from the recorder and the slope of the linear portion of each curve at each increment of load is measured, giving the bob velocity at that load. These velocities are then plotted on linear co-ordinates against their corresponding load units. Because the load units are proportional to the shear stress and the bob velocities are proportional to the shear rate, the curve connecting the plotted points 4 is a rheological diagram of the material under study.

Figure 2 of the drawing shows a chart made by plotting the results of testing several different types of paint on the apparatus described above. From an observation it will be seen that various paints have different bob velocities for a given load increment. The chart shows a rheological diagram of a series of paint samples. It is obvious from an examination of the chart that the samples do not all possess the same flow characteristics. By producing charts of this type, it is possible to determine whether or not a given paint specimen possesses the desirable flow characteristics for a specific purpose without actually applying the paint.

While the invention has been described by referring to the steps taken in analyzing a sample of paint, it will be understood that the instrument is suitable for determining the flow characteristics of any liquid pigmented system, such as inks, adhesives, etc.

It will be obvious from the above that I have developed a. rheometer which is extremely sensitive and which is effective for testing the flow characteristics of a relatively small sample prior to using such a product on a commercial scale.

I claim:

1. An instrument for determining the rheological properties of liquids comprising a receptacle for holding the liquid to be tested, a bob adapted to be suspended freely in said liquid from one end of a balance arm, a linear differential transformer having its freely suspended armature adapted to move with said bob, means for applying a known force to the bob to cause displacement thereof in a vertical direction, said linear differential transformer being adapted to produce an electrical signal proportional to the amount of displacement of the bob, and means for measuring the velocity and magnitude of the displacement of the bob as a function of time at the desired sensitivity.

2. An instrument for determining the rheological properties of liquids comprising a receptacle for holding the liquid to be tested, a bob adapted to be suspended freely in said liquid from one end of a balance arm, a counterweighing device attached to the other end of the balance arm, a linear differential transformer having its freely suspended armature adapted to move with said bob, means for applying a known force to the bob to cause displacement thereof in a vertical direction, said linear differential transformer being adapted to produce an electrical signal proportional to the amount of displacement of the bob, and a recorder having a constantly moving chart to record the intensity of the signal produced by the linear differential transformer as a function of time at the desired sensitivity.

3. An instrument for determining the rheological properties of liquids comprising a receptacle for holding the liquid to be tested, a bob adapted to be suspended freely in said liquid from one end of a balance arm, a counterweighing device attached to the other end of the balance arm, a linear differential transformer positioned between the balance arm and the bob with the freely suspended armature of the transformer connected to the balance arm and the bob, means for applying a known force to the bob to cause displacement thereof in the vertical direction, said linear differential transformer being adapted to produce an electrical signal proportional to the amount of displacement of the bob, and a recorder having a constantly moving chart to record the intensity of the signal produced by the linear differential transformer as a function of time at the desired sensitivity.

4. An instrument for determining the rheological properties of liquids comprising a receptacle for holding the liquid to be tested, a bob adapted to be suspended freely in said liquid from one end of a balance arm, a counterweighing device attached to the other end of the balance arm, a linear differential transformer having its freely suspended armature adapted to move with said bob, means for adjusting the position of the transformer With respect to the armature, means for applying a known force to the bob to cause displacement thereof in a vertical direction, said linear differential transformer being adapted to WILLIAM PATTERSON, JR,

REFERENCES CITED The following references are of record in the file of this patent:

UNITED STATES PATENTS Number Name Date 1,604,409 Grindrod Oct. 26, 1926 1,894,369 Dufling Jan. 17, 1933 

